AlternativeTo - AlternativeTo lets you find apps and software for Windows, Mac, Linux, iPhone, iPad, Android, Android Tablets, Web Apps, Online, Windows Tablets and more by recommending alternatives to apps you already know.
Top Similar Sites - Top Similar Sites helps you find similar, alternative or related sites to any website instantly.
SimilarSites - Easily find more sites like those you like.
Growing Page - Browse web projects and evaluate them by multiple criteria.
Similar Site Search - SimilarSiteSearch helps you find similar, related, or alternative websites.
PitchWall - Browse new startups, and submit your mobile app/games startup's beta page here.